The present invention relates to an image display apparatus such as an HMD (Head-Mounted Display), especially an image display apparatus having an optical system including a diffractive optical element.
The image display apparatus such as an HMD is required to be capable of showing an image at a wide view angle and to have a small-sized and light-weight configuration which does not give an uncomfortable feeling or a feeling of weariness to an observer even if the observer mounts it on his/her head. However, when the view angle increases, the size of the optical system generally increases exponentially and aberration correction becomes difficult to be achieved.
A Single prism optical system for the HMD using internal total reflection and free-formed surfaces has been proposed in Japanese Patent No. 2911750. In this optical system, a single prism introduces a light flux from an image-forming element such as a liquid crystal panel to an exit pupil where an eyeball of an observer is placed through transmission and refraction at an entrance surface (first surface), internal total reflection at a second surface, back surface reflection at a third surface, and transmission and refraction at the second surface as a fourth surface. In this optical system, since the single prism is formed of a single material, chromatic aberration of magnification generated due to dispersion of the prism material when the light flux is transmitted through its transmissive surface cannot be corrected sufficiently.
On the contrary, HMDs have been disclosed in which a holographic element (see Japanese Patent Laid-Open No. 2002-318366) or a diffractive optical element (DOE: see Japanese Patent Laid-Open No. 09-65246)) is disposed between a prism and a lens for the purpose of achromatizing.
In the HMD disclosed in Japanese Patent Laid-Open No. 2002-318366, it is necessary to use glass for a material having dispersion which is capable of obtaining an achromatizing effect, which results in increase in weight of the HMD. In the HMD disclosed in Japanese Patent Laid-Open No. 09-65246, although using the DOE enables effective achromatizing for light of a specific wavelength region, it is difficult to suppress unnecessary diffraction order light from being generated in the entire view angle region for light of a wide wavelength region forming a color image.
The DOE has a negative dispersion due to its characteristics. Therefore, although the chromatic aberration of magnification generated in a general optical material can be efficiently corrected, a diffraction efficiency of 100% or close thereto cannot be achieved under conditions other than a specific wavelength region and a specific incident angle, so that unnecessary diffraction order light is generated. In this case, when an image with a high contrast is displayed, flare is generated which deteriorates image quality of the displayed image or a contrast of a high frequency image is decreased.
In contrast thereto, a method for achieving a high diffraction efficiency in a wide wavelength region has been disclosed in Japanese Patent Laid-open Nos. 09-127321 and 09-127322. In this method, plural DOEs which are produced by mutually different materials to have mutually different design diffraction orders are mutually closely arranged.
Further, examples of correcting the chromatic aberration of magnification by using DOE in the optical system for the HMD have been disclosed in Japanese Patent Nos. 3482396 and 3559624, and Japanese Patent Laid-open No. 2002-311380.
In the HMD disclosed in Japanese Patent No. 3482396, a prism having three surfaces including a transmissive surface, a transmissive/reflective surface and a reflective surface folds an optical path of a light flux from an image-forming element in the prism to introduce the light flux to an exit pupil. The HMD is provided with the DOE having the function of correcting the chromatic aberration of magnification at any position between the image-forming element and the exit pupil.
In the HMD disclosed in Japanese Patent No. 3559624, a decentered prism is used which has three surfaces including at least one rotationally asymmetric formed surface, and the DOE formed on a parallel plate is disposed between the image-forming element and the prism.
Moreover, the HMD disclosed in Japanese Patent Laid-Open No. 2002-311380 is provided with a positive lens between the decentered prism and the exit pupil in order to enlarge the view angle. In addition thereto, to correct the chromatic aberration of magnification increased due to the lens, the DOE is provided on any surface provided in the optical path.
As described above, the use of the DOE enables correction of the chromatic aberration of magnification in optical design of the HMD. However, the following matters must be further considered due to a particular condition for the HMD in which the eyeball of the observer is placed at a position of the exit pupil.
In the HMD, a rotational position of the eyeball of the observer (that is, visual axis) does not always coincide with the optical axis of the optical system. In an optical system using only refraction and reflection, aberrations are corrected by increasing a diameter of the exit pupil as much as possible such that the entire displayed image can be observed favorably even though such incoincidence is occurred. Therefore, even though there exists a certain displacement between the visual axis of the observer and the optical axis of the optical system, serious problems do not occur in observing the image.
On the other hand, the design of the DOE used in the optical system is optimized for light rays reaching respective points on the DOE at a specific incident angle. However, when a pupil of the eyeball is displaced with respect to a center of the exit pupil such that the incident angle of the light rays reaching the DOE is significantly deviated from the specified incident angle, unnecessary diffraction order light is generated, thereby deteriorating image quality of the displayed image.
Particularly, providing a sufficient optical power to the DOE to correct chromatic aberrations reduces a pitch of the diffraction grating, which significantly increases the unnecessary diffraction order light. Japanese Patent Nos. 3482396 and 3559624, and Japanese Patent Laid-Open No. 2002-311380 do not describe about these problems.
In the case where the DOE is used for the HMD, it is desirable that the DOE is disposed at a position close to the exit pupil in the entire optical system from a viewpoint of assembly of the HMD and a space for disposing the DOE. Further, the optical system for the HMD often uses a decentered optical system and a rotationally asymmetric formed surface. However, it is difficult to form the DOE on an optical surface in the decentered optical system and the rotationally asymmetric curved surface because the surfaces have a complicated shape.
Moreover, even though a planar DOE is provided between these optical surfaces or between the image-forming element and the prism, the shape of the DOE becomes complicated, so that producing the DOE becomes difficult.
Further, the number of options of the image-forming elements having a small original image size is small, and it is necessary to keep an enlarging magnification ratio of the displayed image with respect to the original image at a certain ratio or lower. Therefore, if the DOE is disposed between the image-forming element and the prism, freedom of design of the optical system is reduced because of providing the space for disposing the DOE.
In contrast thereto, if the optical system is designed such that the DOE is disposed at a position closest to the exit pupil (eyeball of the observer) and the optical system does not include a decentered surface having an optical power between the DOE and the exit pupil, the DOE can be formed so as to have grating rings having a rotationally symmetric shape on a planar base member, which facilitates production of the DOE.
However, as mentioned above, if the visual axis of the observer does not coincide with the optical axis of the optical system, the incident angle onto the DOE of each light ray emerging from each point on the image-forming element, passing through the DOE and then reaching the eyeball is displaced with respect to the designed incident angle. In this case, a short distance from the eyeball to the DOE increases the difference between an actual incident angle of the light ray onto the DOE and the designed incident angle, which increases the unnecessary diffraction order light. If the DOE is provided with an optical power only for the achromatizing, the grating pitch at a high view angle reduces, thereby increasing the unnecessary diffraction order light more easily.
Such incoincidence between the visual axis of the observer and the optical axis of the optical system also occurs when the HMD is mounted on the head in a state where the exit pupil of the HMD is displaced with respect to the eyeball of the observer.
Thereafter, particularly when the DOE is disposed near the exit pupil, the optical system for the HMD requires optical measures to reduce the unnecessary diffraction order light caused by incoincidence between the visual axis of the observer and the optical axis of the optical system.